Many types of electronic systems include a receiver that must process a received signal. The signal may be applied to a carrier wave, such as by adjusting or modulating the amplitude of the carrier wave. Amplitude-Modulation (AM) and Amplitude-Shift-Keying (ASK) are two methods to modulate an amplitude to carry a signal.
A receiver may use an envelope detector to extract the signal from the carrier wave. The envelope detector outputs a signal that generally follows the peaks in the carrier wave over time. The envelope signal may then undergo further processing, such as by a demodulator or a digital-signal processor (DSP).
FIG. 1 is a graph of envelope detection. Carrier wave 102 has a generally constant frequency but its amplitude is modulated to carry a signal. The signal carried may contain or represent various information, such as a song, music, data, video, encrypted data, or some other kind of data stream.
An envelope detector ideally generates an upper envelope signal 104 from the positive peaks of carrier wave 102, and lower envelope signal 106 from the negative peaks or troughs of carrier wave 102. However, circuit losses in a real envelope detector may produce a voltage drop or loss, so that upper envelope real output 108 is lower in voltage than upper envelope signal 104. Similarly, lower envelope real output 110 has a smaller absolute voltage than lower envelope signal 106.
These circuit loses may be caused by voltage drops from a diode rectifier, a filter such as an R-C filter that imposes an R-C time constant limit, and various impedances that are sensitive to frequency. For example, a simple envelope detector having a diode rectifier followed by a filter has a maximum frequency that is limited by the filter's R-C time constant. Ripple on a power or ground line may disrupt the stability of some envelope detectors, such as those based on transistor inverters or drivers. Transistors having a grounded source or drain may inject power-line or ground ripple into the detected signal. Schmidt-trigger stages may not operate over a wide range of input voltages or signals with large swings. Active circuits such as opamps, equalizers, differentiators, and Phase-Locked Loops (PLL's) increase circuit complexity and may introduce secondary problems such as harmonics and loop stability.
Prior-art envelope detectors are often sensitive to frequency. As the frequency of carrier wave 102 or of the data being carried increases, the impedance losses and voltage drop can increase significantly. As frequency rises, the voltage drop can approach the total amplitude of carrier wave 102, creating an upper frequency limit to the envelope detector and the receiver. Thus very high data bit rates may not be allowed. Very complex circuits to tune the phases of recovered clocks may be needed, but these complex circuits may impose their own limits to operating frequency due to their complexity. Jitter and phase errors may increase and cause problems. Getting the clock phase to exactly match the peak in carrier wave 102 may be quite difficult.
What is desired is an envelope detector that can operate at very high frequencies and data rates. An envelope detector circuit that does not have many active components such as amplifier transistors with powered or grounded sources or drains is desirable to reduce ripple problems. An envelope detector with time-multiplexed or parallel paths is desirable to increase data rates.